The field of this invention is power swivel assemblies and the like.
Power swivel assemblies and assemblies having power subs coupled to conventional drilling swivels are most frequently used in oil field operations requiring the rotation of pipe. While there are a number of such operations, power swivels and power subs are particularly useful for rotating casing during downhole cementing operations.
Known power swivels and power subs typically included a rotatable stem, an upper end of which was coupled with a swivel member. The stem was rotated by a motor and a gear train interconnecting the motor to the stem. The gear train often included a bull gear rigidly affixed to the stem and extending radially therefrom to engage a mating drive gear mounted on the drive shaft of the motor. The motor was spaced radially from the stem to facilitate alignment of the drive gear and the bull gear.
While such known power swivels and power subs did achieve a rotation of the stem, there were several disadvantages attendant to their use. Because the motor was radially spaced from the stem, such assemblies often did not have a center of gravity along their longitudinal axis. This displacement of the center of gravity made the unit difficult to handle. Additionally, the center of gravity of the assemblies could not readily be aligned with the center line of the borehole. As a result, the assemblies exerted at least some torque on the stem and on the pipe being rotated. This was particularly disadvantageous when the assembly was being used to run flexible, lightweight tubing. The radial spacing of the motor from the stem also exposed the motor to damage by being hit with other apparatus being brought to, removed from, or otherwise utilized on the rig platform. Further, the positioning of the motor reduced the working space available on the rig platform. An additional disadvantage of the known assemblies related to the use of the interconnecting gear train to rotate the stem. Such gear trains were somewhat expensive, required maintenance, and resulted in some power loss in the transmission of power from the drive motor to the rotatable stem.